DC voltage is converted into AC voltage and direct current is converted into alternating current usually using suitable inverters designed as switched-mode converters. For conversion, conventional inverters comprise a switching network with a plurality of semiconductor switches that are switched on (i.e. switched to low impedance) and switched off (i.e. switched to high impedance) at high frequency by means of appropriate actuation. In this case, for the purpose of signal shaping for the alternating current, the semiconductor switches are usually actuated digitally and using actuating signals that have a clock frequency and a pulse width modulation (PWM) overlaid on this clock frequency. In this case, a desired sinusoidal shape of the alternating current to be supplied to the AC mains is approximated all the better the higher the clock frequency for the conversion of the direct current into alternating current. However, the increase in switching losses means that the efficiency of the inverter usually falls as the clock frequency increases. Although the efficiency of the inverter rises when the clock frequency decreases, the desired sinusoidal shape of the alternating current to be supplied is approximated increasingly poorly and the proportion of harmonics that are overlaid on the alternating current to be supplied increases.
For the supply of alternating current to a public AC mains designed for a power distribution, limit values that specify a permitted proportion of harmonics overlaid on the pure sinusoidal shape for supply are defined on a normative basis. The limit values are usually specified on both a frequency-dependent and a cumulative, i.e. summed over all frequencies, basis and are cited in appropriate country-specific standards using the term THD (THD=Total Harmonic Distortion).
So as now not to exceed the limit values when supplying alternating current to a public power distribution grid (EVN), conventional converters have a passive filter. The passive filter, which usually has inductances and capacitances, provides a high impedance for high frequency interference signals overlaid on the sinusoidal shape and thereby reduces the supply of the interference signals to the AC mains. Passive filters of this kind are expensive, however, and, specifically in the case of inverters in low power classes (usually with a rated power Prated≤5 kW), contain a not insignificant proportion of the overall costs of the inverter.
A small frequency spacing between the interference signal to be filtered out and the actual useful signal—particularly the frequency of the AC mains of e.g. 50 Hz—requires higher-order passive filters, which have a more complex design and also higher costs. However, higher-order passive filters also still have a certain proportion of undesirable attenuation of the useful signal that increases as the frequency spacing between interference signal and useful signal decreases. In addition, passive filters have only restricted suitability for selective filtering of common-mode and differential-mode signals. In any case, conventional inverters designed as switched-mode converters require complex signal conditioning in order to reject the supply of interference signals overlaid on the alternating current to be supplied.
To reduce harmonics in a power distribution grid, active filters are used. Corresponding active filters are described by way of example in the technical article “Clean Grid Solutions” from the Danfoss company under the Internet link http://danfoss.ipapercms.dk/Drives/DD/Global/SalesPromotion/Brochures/ProductBrochures/DE/CleanGrid/. Such filters can be connected to the power distribution grid in parallel with a nonlinear load. These active filters are modified frequency converters with feedback capability that detect harmonic currents present in the power distribution grid and feed back currents phase-shifted through 180° to the power distribution grid. The currents supplied are overlaid with the harmonic currents present in the power distribution grid and thus result in cancellation of the harmonic currents therein. In this case, the power distribution grid is thus initially burdened by an interference current, but this is compensated for again as promptly as possible by the active filter by means of phase-offset supply of current.
DE 10 2014 101 571 A1 discloses an inverter for supplying electric power from a direct current source to an AC mains. In the event of the supply of reactive power during periods of time from a system period of the AC mains that involve the inverter drawing electric power from the AC mains, control of switches of the inverter bridge converts the electric power drawn from the AC mains into heat. This involves the control operating a first switch of the inverter bridge in a linear or a dissipatively clocked mode of operation. This prevents a flow of power from the inverter bridge in the direction of the direct current source.
The documents EP 0578548 A1 and AT 71779 B are disclosing an uninterruptable power supply connected between an AC-mains and a consumer and containing digitally operated as well as linearly operated semiconductor switches.
The document DE 102009 029387 A1 discloses a DC/AC inverter assembly including a semiconductor bridge circuit, wherein a DC/DC converter is provided for creating half-waves of an AC voltage on the output side. The bridge circuit is connected downstream of the DC/DC converter and acts as pole changer on the half-waves